Combustion exhaust gases of coal, heavy oil, etc., combustion exhaust gases of municipal waste or industrial waste, gases generated by the pyrolysis of plastics and so on contain various compounds, such as nitrogen oxides, sulfur oxides, aromatic compounds, chlorine-containing organic compounds, chlorinated aromatic compounds and other halogen-containing compounds, although their contents are minor. In many cases, two or more of them coexist, i.e. exist in a mixed state. As a rapid measurement technique of these compounds, there is a method of laser multiphoton ionization mass spectrometry which has detection selectivity of the compounds to be measured.
An example of the technique for measuring a mixed gaseous sample by laser multiphoton ionization mass spectrometry is disclosed in Analytical Chemistry, vol. 66, pp 1062-1069, 1994. That is, according to the laser multiphoton ionization mass spectrometry having a conventional sample introduction system, peaks corresponding to each compound overlap with each other due to broad peaks, and therefore, quantitative analysis is difficult. Thereupon, a gaseous sample is introduced into a vacuum ionization chamber through a sample inlet valve having a small bore diameter. The gaseous sample is ionized by irradiating a laser beam, and measured by a mass spectrometer. At that time, since the gaseous sample is cooled to near zero degree of absolute temperature by adiabatic expansion, vibration and rotation of the molecules of each compound are inhibited. Accordingly, peaks corresponding to each compound are rendered sharp and separated from each other, resulting in the facilitation of quantitative analysis. Since the speed of the introduced molecules is about tens of times as much as sonic velocity, this method is also called supersonic molecular beam spectroscopy or supersonic molecular jet spectroscopy. In the document, it is described that a standard laser beam irradiation time is 10 ns.
In the sample introduction in the supersonic molecular jet, in general, there is a restriction of the introduction amount per unit time of a gaseous sample introduced continuously or intermittently in order to maintain high vacuum conditions of the ionization chamber. As a result, it is a problem that sensitivity on the whole is lowered due to a very small amount of sample to be measured. As a countermeasure, it has been considered to increase the laser beam irradiation energy.
However, when the laser beam energy is increased, a problem occurs that accurate determination cannot be made because of decomposition, i.e. fragmentation, of the molecules to be measured.
Another countermeasure against the above sensitivity reduction caused by minor amount of measuring objects, there is a method of introducing a sample at a laser beam passage by using a slit-shaped nozzle, as disclosed in Review Science Instrumentation, vol. 67, pp 410-416, 1996. In the method, the laser beam is irradiated perpendicular to a molecular jet jetted planar on the same plane, and thereby, the interacting space between molecules and laser beam increases to increase the production of ions.
Although this method is effective, however, in principle, ions are produced and exist in the space in proportion to the size of the slit opening and the diameter (size) of the irradiating laser beam. That is, to enlarge the slit opening relates to the space distribution of molecular ions being delivered in a mass spectrometer, and it does not contribute to the increase of signal/noise ratio (S/N ratio) by a portion in proportion to ion production. Moreover, the load on the exhaust system must be considered, and accordingly, the slit opening cannot be enlarged to an extreme. That is, only the central portion along the major axis of the slit opening contributes to the signal, and molecules existing on the periphery do not contribute to the signal, and, nevertheless, lower the degree of vacuum. Furthermore, since the cooling of the molecular jet is inferior, there is a possibility to lower the S/N ratio conversely. These matters are also problems induced by enlarging an opening for the purpose of introducing a sample in quantity, not only in a slit nozzle but also in a pinhole nozzle.
Accordingly, unless the sample flow is increased, the sensitivity cannot be improved. If sample flow is increased, it lowers the degree of vacuum of the mass spectrometer at a later stage to stop the mass spectrometer by working a safeguard for apparatus protection. This problem is especially remarkable when a measurement is conducted near full capacity of the exhaust system with pulse injection of a sample which brings a great pressure variation, in order to improve sensitivity.
On the other hand, in order to make the supersonic molecular jet, an ionization chamber, related portions thereto, and so on must be made in high vacuum conditions, and in general, a diffusion pump, i.e. oil diffusion pump, is frequently used for the exhaust system, as disclosed in Ed. by The Chemical Society of Japan, "Jikken Kagaku Koza", 4th Ed., vol. 8, p119, 1993.
An oil rotary pump or the like or a combination of both pumps are also used. The exhaust velocity of an oil diffusion pump and an oil rotary pump is, in general, high, i.e. high vacuum conditions can be maintained. However, the oil used in the pumps exists in the ionization chamber, although the amount is very small. As a result, it is a problem that the oil is ionized as it is or ionized through a decomposition reaction, by the irradiation of pulsed laser beam, and causes an increase in background noise.
An object of the invention is, in a laser multiphoton ionization mass spectrometry technique with a sample introducing system by a supersonic molecular jet, to provide an apparatus and a method capable of detecting in high sensitivity and measuring stably.
Another object of the invention is to provide an apparatus, although intending to introduce a sample in quantity, capable of detecting in high sensitivity by not lowering a S/N ratio.
Still another object of the invention is, in a laser multiphoton ionization mass spectrometry technique with a sample introducing system by supersonic molecular jet, to provide a measuring apparatus which lowers background noise and thereby increases signal strength.